Acitivity in auditory nerve fibers is conveyed snyaptically to the cochlear nuclei, where the first stages of neural integration of auditory information take place. Several types of cells can be distinguished in the cochlear nuclei that have their own characteristic pharmacology, morphology, inputs, pattern of projection, electrical characteristics and response patterns to tones. Because many of these cells are interconnected and receive efferent inputs from higher centers, the pattern of activity in the cochlear nucleus to the complex spatial and temporal patterns of activity in the auditory nerve that are evoked by natural sounds are not understood. Experiments are proposed to define the neuronal circuitry in the psoteroyentral and dorsal cochlear nuclei. In brain slices, stimulation of the auditory nerve evokes not only monosynaptic responses but also polysynaptic responses through neuronal circuitry contained in the slice. Elements of the circuit will be separated physically, pharmacologically and electrophysiologically. Synaptic responses will be recorded intracellularly to electrical stimulation of the auditory nerve and of other fiber tracts. Circuitry will be simplified by cutting away inputs. The connections that remain will be differentially activated with focal stimulation and antagonists to neurotransmitters will be used to separate synaptic responses that are activated together. The synaptic responses of a single cell will be related to the activity of other cells in the slice through a map of extracellularly recorded, evoked responses. Because the cochlear nuclei encode sound tonotopically, the position of activity in the slice will reflect, in a crude way, the frequency of sound encoded by the electrically stimulated fibers. No natural sounds activate large groups of auditory nerve fibers simultaneously as do auditory prostheses; the limitations of auditory prostheses result from the unnatural patterns of activity evoked in the auditory pathway. The patterns that will be recorded from slices in response to electrical stimulation of the auditory nerve, will be unnatural in precisely the same way as auditory prostheses. By understanding the neuronal circuitry in the cochlear nuclei that underlies these patterns, it may be possible to improve the performance of prostheses by altering critical spatial or temporal stimulation patterns.